1. Field of the Invention
The present invention relates to a projection type display device such as a liquid crystal projector, and more particularly, to a projection type display device which allows an user to readily correct an optical image for distortions which are produced when an optical image is enlarged and projected on a screen with a tilt angle in the vertical and horizontal directions through a projection lens.
2. Description of the Related Art
Conventionally, a projection type display device is typically placed below the center of a screen for projecting an enlarged image with a projection lens oriented upward. However, in this event, the image projected on the screen suffers from an inverse trapezoidal distortion (trapezoidal having an upper side longer than a lower side). In addition, when the projection type display device is placed at an angle in the horizontal direction (lateral direction) to the screen, the projected image further extends toward an upper left side or an upper right side to present a more complicated trapezoidal distortion.
Now, FIG. 1 shows the relationship between the placement of the projection type display device and an image projected on the screen.
Referring to FIG. 1, the image projected on screen 101 reproduces original rectangular image d without tilt distortions when the image is projected from projection lens 102 of projection type display device 100 along projection optical axis a without tilt to screen 101. However, when the image is projected along optical axis b which is vertically tilted, display device 100 provides vertically tilted image e in an inverse trapezoidal shape. Moreover, when the image is projected along optical axis c which is horizontally tilted to the right, the resulting image further extends toward an upper right side to present horizontally and vertically tilted image f which has more complicated distortions.
To consider such a distortion, a correction for a trapezoidal distortion, which is produced when projection type display device 100 is positioned as shown in FIG. 1 (projection lens 102 is oriented upward and tilted to the right), will be described with reference to FIGS. 2A to 2C.
FIGS. 2A to 2C are diagrams for describing how an image is corrected for a trapezoidal distortion.
FIGS. 2A, 2B illustrate image 113 displayed within image display range 115 on an image display device (liquid crystal panel) when a vertical correction is made. Displayed image 113 on an image display device (not shown) is distorted in a direction opposite to an inverse trapezoidal distortion of the image 101 projected on the screen to correct the projected image for the distortion, resulting in a rectangle. Though not shown, the correction for a vertical distortion involves increasing a compression ratio of a line corresponding to the upper side of the projected image on the displayed image, on the image display device, and reducing the compression rate toward the lower side.
FIGS. 2B, 2C illustrate image 113 displayed on the image display device when it is subjected to a horizontal correction after the vertical correction.
In this example, a transition from FIG. 2A to FIG. 2B implies movements of upper left vertex P101 and upper right vertex P102, and a transition from FIG. 2B to FIG. 2C implies movements of lower left vertex P103 and lower right vertex P104, eventually resulting in movements of all four vertexes P101, P102, P103, P104.
When an user corrects an image for distortions, the user typically matches the vertexes of the image with the vertexes of screen 101 before the correction for the distortion, so that the vertexes of the image move each time a vertical correction or a horizontal correction is made as illustrated in FIG. 2A to 2C, resulting in difficulty in correction for the distortion.
Even if the projection type display device is equipped with an acceleration sensor for measuring a tilt angle in the vertical direction to automate the vertical correction, difficulties in the correction for distortion are not mitigated because all the four vertexes still move in the horizontal correction.
To solve this problem, Japanese Patent Laid-open Publication No. 198993/2003, for example, discloses a projection type display device which collectively corrects an image for distortion produced when the image is projected at tilt angles in two directions, i.e., for a trapezoidal distortion produced when the image is projected at an tilt angle in the vertical direction, and for a trapezoidal distortion when the image is projected at an tilt angle in the horizontal direction. This projection type display device comprises a tilt sensor for detecting a tilt angle in the vertical direction, and a rotary knob manipulated by the user to correct an image for distortions in the horizontal direction. The result detected by the tilt sensor is applied to a distortion correcting circuit through a microcomputer to automatically correct the image for distortions in the vertical direction. Together with this correction, a voltage which varies in accordance with rotating angles of the rotary knob, is applied to the microcomputer which converts the voltage to data related to a horizontal tilt angle set by the user, and the data is applied to the distortion correcting circuit. In this way, by rotating the rotary knob by an angle equivalent to the horizontal tilt angle of the projection type display device in the reverse direction, the image caused by the horizontally tilted projection is corrected for distortions in the horizontal direction, with an intuitive easy-to-understand adjustment.
In the foregoing projection type display device, a correction is made by rotating in the reverse direction the scaled rotary knob by an angle equal to an angle by which the display device is tilted in the horizontal direction. However, since the user must make corrections while watching both the scale on the rotary knob and the image projected on the screen, and each correction causes the vertexes to move, the correction of an image for distortions entails difficult manipulations, so that an user must attain proficiency in the manipulations.